Gas permeable hard contact lens and method of producing it

ABSTRACT

Gas-permeable hard contact lenses are produced by hot press-stretching a crosslined gas-permeable hard contact lens material and then machining the press-stretched material. The gas-permeable hard contact lenses are formed from a crosslinked gas permeable hard contact lens material which is hot press-stretched to have a compression ration of 5 to 50% and a compression-flexure fracture strength of 300 to 1,500 g. Efficiently produced gas-permeable hard contact lenses are produced which are free of optical strains, excellent in transparency, small internal stress and comfortable to wear with improved durability strength.

TECHNICAL FIELD

The present invention relates to a process for the manufacture of agas-permeable hard contact lens and a gas-permeable hard contact lens.More specifically, it relates to a process for efficiently producing agas-permeable hard contact lens having excellent durability strength anda gas-permeable hard contact lens which is improved in durabilitystrength and free of optical strains and which has no problem onwearing.

TECHNICAL BACKGROUND

A contact lens is generally classified into a hard contact lens and asoft contact lens. Further, the hard contact lens is classified into anoxygen-non-permeable hard contact lens formed of a homopolymer orcopolymer of methyl methacrylate (MMA) and a rigid gas permeable (RGP)hard contact lens formed of a copolymer formed of siloxanyl methacrylate(SiMA), MMA and fluoroalkylmethacrylate (FMA) as main components.

In the beginning, the main stream of a hard contact lens was anoxygen-non-permeable hard contact lens formed of polymethyl methacrylate(PMMA) having excellent biocompatibility and excellent transparency.Concerning the above oxygen-non-permeable hard contact lens, however, ascontact lenses have come to be used widely, damage is caused on cornealepithelium since the wearing time period thereof is extended, orinfluences which the long wearing time period thereof has on cells ofcorneal endothelium have come to be discussed. Hard contact lenses withmore safety have come to be developed.

The gas-permeable hard contact lens is a hard contact lens developedunder the above circumstances, and it is classified into alow-oxygen-permeable type and a high-oxygen-permeable type dependingupon an oxygen permeability coefficient (DK value). In present markets,the main stream is a high-oxygen-permeable type (continuous-wearingcontact lens) due to a further extended wearing time period.

Since, however, the gas-permeable hard contact lens has molecular-levelholes for allowing oxygen gas necessary for cornea wearing the contactlens to permeate therethrough and exhausting metabolically generatedcarbon dioxide gas, it has a defect that its durability strength againstan instantanious impact or bending decreases inevitably with an increasein the DK value.

For overcoming the above defect, there is proposed an oxygen-permeablehard contact lens which has higher compression fracture strength than aconvention contact lens and is not easily broken (JP-A4-67117). In theabove method, however, it is required to carry out a polymerization soas to attain a uniform polymerization not only by controlling thepolymerization rate of monomers used but also by precisely controllingpolymerization conditions. It is therefore very difficult to control thepolymerization conditions.

There is also proposed a methods in which a non-crosslinked polymerformed by polymerizing monomers containing at least one monomer selectedfrom an unsaturated carboxylic acid, an unsaturated carboxylate ester oran unsaturated carboxylic acid anhydride is compression-molded underheat in the presence of a polyamine, to produce a transparent opticalresin molded article improved in solvent resistance and mechanicalstrength (JP-A-7-62022). In the above method, the molded articleobtained in excellent solvent resistance, free of optical strains,homogeneous and excellent in transparency since a non-crosslinkedpowdery polymer is crosslinked in the presence of polyaminde during thecompression molding under heat, and further, it undergoes almost nochange of a form with the passage of time since the internal stressgenerated in the polymer after the molding is very small. However, dueto a diversity of base curves of hard contact lenses and due to avariety of diopters and a variety of diameters, the above method has adefect that the number of molds therefor increases and that it requiresimmense labor to manage them.

Further, there is proposed a method in which a contact lens material iscompression-molded under heat to produce a contact optical moldedarticle (JP-A-60-49906 and JP-A-61-41118). In this method, a film havinga weight of a molded article to be produced and having a uniformthickness is punched out or cut, and the resultant piece is placebetween dies having convex surfaces correspond to the form of a moldedarticle or dies having convex and concave surfaces and a re-molded underpressure at a temperature higher than the glass transition temperatureof a thermoplastic material used but lower than the melt-flowingtemperature thereof, to produce a contact optical molded article. In theabove method, however, a film-shaped non-processed produce which isformed of a thermoplastic resin having no crosslinked structure and hasa weight equivalent to the weight of a molded article is molded to afinished form, and the molded article obtained has a problem that theform thereof changes with the passage of time or that the strengththereof is not sufficient since it has no crosslinked structure.

Supplying oxygen to cornea wearing a contact lens naturally alleviates aphysiological burden on the cornea, and it is said that the deficientlyof oxygen in cornea has a clinically large influence. A contact lens isrequired to secure sufficient safety for a long period of time since itis to be in direct contact with a living tissue, cornea, which is highlysensitive and essential for the function of vision. Further, sincecornea constantly requires oxygen for maintaining transparency, acomposition of monomers to be contained, or the like, is devised forincreasing the DK value. However, when the DK value is increased, thereis caused a problem that the durability decreases. Further, copolymerswhich are improved in durability strength by adding a strength-impartingmonomer or a crosslinked agent are available, while they cannot be saidto be satisfactory.

DISCLOSURE OF THE INVENTION

Under the circumstances, it is a first object of the present inventionto provide a process for efficiently producing a gas-permeable hardcontact lens which is improved in durability strength, is free ofoptical strains and has no problem on wearing.

It is a second object of the present invention to provide agas-permeable hard contact lens which is improved in durabilitystrength, is free of optical strains and has no problem on wearing.

For achieving the above objects, the present inventors have madediligent studies and as a result have found that the above first objectcan be achieved by hot press-stretching a crosslinked gas-permeable hardcontact lens material and then machining it.

Further, it has been found that the second object can be achieved by agas-permeable hard contact lens obtained by the above process and agas-permeable hard contact lens formed of a crosslinked gas-permeablehard contact lens material which is hot press-stretched so as to have acompression ratio and a compression-flexure fracture strength inspecific ranges.

The present invention has been made on the basis of the above findings.

That is, the present invention provides:

(1) a process for the production of a gas-permeable hard contact lens,which comprises hot press-stretching a crosslinked gas-permeable hardcontact lens material and then machining the press-stretched material,

(2) a gas-permeable hard contact lens obtained by the above process (1),and

(3) a gas-permeable hard contact lens formed of a crosslinkedgas-permeable hard contact lens material which is not press-stretched tohave a compression ratio of 5 to 50% and a compression-flexure fracturestrength of 300 to 1,500 g.

BRIEF DESCRIPTION OF DRAWINGS

The figures are partial schematic views of different examples of heatpress-stretching apparatus used for practicing the process of thepresent invention.

FIG. 1 illustrates a press for heat press-stretching a button-shapedcontact lens material,

FIG. 2 illustrates a press for heat press-stretching a rod-shapedcontact lens material,

FIG. 3(a) is a cross-section of a rod-shaped contact lens material in ajig in contact with a press member before heat press-stretching, and

FIG. 3(b) is a cross-section of a rod-shaped contact lens materialduring heat press stretching.

BEST MODES FOR PRACTICING THE INVENTION

In the process for the production of a hard contact lens provided by thepresent invention, a crosslinked gas-permeable hard contact lensmaterial is used as a material. The lens material is not speciallylimited, and it can be properly selected from known lens materialsconventionally used for gas-permeable hard contact lenses. The abovecrosslinked gas-permeable hard contact lens material is preferably alens material formed of a copolymer containing fluorine-containing(meth)acrylate and silicon-containing (meth)acrylate as main componentsand containing a crosslinking monomer, a hydrophilic monomer and asiloxane oligomer having a polymerizable functional group in a molecularterminal. In the present invention, “(meth)acrylate” stands for acrylateor methacrylate.

The above fluorine-containing (meth)acrylate preferably includefluoroalkyl (meth)acrylates such as 2,2,2-trifluoroethyl (meth)acrylate,2,2,2,2′,2′,2′-hexafluoroisopropyl (meth)acrylate,2,2,3,3,4,4,4-heptafluorobutyl (meth)acrylate,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafulorooctyl (meth)acrylate and2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-hexadecafluorononyl (meth)acrylate. Ofthese, fluoroalkyl methacrylates are preferred. Thesefluorine-containing (meth)acrylates may be used alone or in combination.

The silicon-containing (meth)acrylate includes siloxanyl (meth)acrylatessuch as tris(trimethylsiloxy)silylpropyl (meth)acrylate,heptamethyltrisiloxanylethyl (meth)acrylate, pentamethyldisiloxanyl(meth)acrylate, isobutylhexamethyltrisiloxanyl (meth)acrylate,methyldi(trimethylsiloxy)-(meth)acryloxymethylsilane,n-propyloctamethyltetrasiloxanylpropyl (meth)acrylate,pentamethyldi(trimethylsiloxy)-(meth)acryloxymethylsilane andt-butyltetramethyldisiloxanylethyl (meth)acrylate, and further includestrimethylsilyl (meth)acrylate and phenyldimethylsilylmethyl(meth)acrylate. Of these, siloxanyl (meth)acrylates are preferred, andsiloxanyl methacrylates are particularly preferred. Thesesilicon-containing (meth)acrylates may be used alone or in combination.

The above fluorine-containing (meth)acrylate and the abovesilicon-containing (meth)acrylate have a high effect on improving theoxygen permeability of a copolymer, and when these two (meth)acrylatesare used in combination, there can be obtained a copolymer havingdesired oxygen permeability. The oxygen permeability of the copolymercan be adjusted by adjusting the mixing ratio of these two(meth)acrylates.

The content ratio of a fluorine-containing (meth)acrylateunit:silicon-containing (meth)acrylate unit in the copolymer differsdepending upon desired oxygen permeability. Generally, the weight ratiothereof is in the range of from 70:30 to 40:60, preferably in the rangeof from 60:40 to 50:50.

The crosslinking monomer includes alkylene glycol di(meth)acrylates suchas ethylene glycol di(meth)acrylate and diethylene glycoldi(meth)acrylate, and difunctional or higher monomers such astrimethylolpropane tri(meth)acrylate and pentaerythritol tetra- ortri(meth)acrylate. These crosslinking monomers may be used alone or incombination. The above crosslinking monomer has an effect on impartingthe copolymer with hardness and chemical resistance.

The content of the crosslinking monomer in the above copolymer isgenerally in the range of from 0.1 to 20% by weight.

The hydrophilic monomer includes hydroxyl-group-containing(meth)acrylates such as 2-hydroxyethyl (meth)acrylates, 2-hydroxypropyl(meth)acrylate and 2-hydroxybutyl (meth)acrylate, unsaturated carboxylicacids such as acrylic acid, methacrylic acid, itaconic acid, fumaricacid, maleic acid and sinnamic acid, (meth)acrylamides such asacrylamide, methacrylamide, dimethylacrylamide and dimethylacrylamide,vinylpyridine and vinylpyrrolidone. These hydrophilic monomers may beused alone or in combination. The above “(meth)acrylamides” stand foracrylamides or methacrylamides.

When used as a comonomer, the above hydrophilic monomer has the effectof improving the copolymer in wetting properties, and it improves a lensin its affinity to tears when the lens is worn so that it improves thewearing feeling. An unsaturated carboxylic acid is particularlypreferred since it has remarkable effects on improving the copolymer inhardness and improving the copolymer in wetting properties.

The content of the above hydrophilic monomer unit in the copolymer isgenerally in the range of from 5 to 20% by weight.

The siloxane oligomer having a polymerizable functional group in amolecular terminal is used for improving the copolymer in impactresistance. For example, it is preferred to use a compound of thegeneral formula (I),

wherein m is an integer of 5 to 200, and each of A and A′ is a group ofthe general formula,

in which R is hydrogen atom or methyl, and may be the same as the otheror different from the other.

The above siloxane oligomer preferably has a molecular weight in therange of from approximately 500 to 15,000, and isophoronediisocyanate-based siloxane oligomer [oligomer of the general formula(I) in which A and A′ are groups represented by (b)] is preferred due toits remarkable effect of improving the impact resistance.

The content of the above siloxane oligomer component in the copolymer isgenerally in the range of from 0.1 to 15% by weight.

The crosslinked gas-permeable hard contact lens material used in thepresent invention can be prepared, for example, by a method shown below.

A polymerization initiator such as azobisisobutyronitrile,azobisdimethylvaleronitrile, benzoyl peroxide or lauroyl peroxidepreferably in an amount of 0.05 to 1% by weight is added to, and mixedwith, a monomer mixture containing the above fluorine-containing(meth)acrylate, silicon-containing (meth)acrylate, crosslinking monomer,hydrophilic monomer and siloxane oligomer, and then, the resultantmixture is poured into a casting container formed of a metal, plastic orglass and polymerized under heat after the container is closed, toprepare a circular button-shaped or rod-shaped crosslinked gas-permeablehard contact lens material. For the polymerization, ultravioletpolymerization may be employed beside the heat polymerization.

In the process of the present invention, the above-prepared circularbutton-shaped or rod-shaped contact lens material (to be sometimesabbreviated as “button material” or “rod material” hereinafter) is hotpress-stretched so as to have a thickness smaller than the thickness thematerial has before press-stretched. The thickness after thestretch-pressing may be a magnitude of the button material or the rodmaterial that can be processed in a general hard contact lens processingstep, while it is preferably a magnitude in which the compression ratiois 5 to 50%. The compression ratio is calculated on the basis of thefollowing equation.

Compression ratio (%)=[(Height (mm) of material beforecompression−height (mm) of material after compression)/height (mm) ofmaterial before compression ]×100

Although differing depending upon contact lens materials, the buttonmaterial or the rod material during the hot press-stretching generallyhas a temperature in the range of from 100 to 140° C.

The method of the hot press-stretching will be explained below.

When the contact lens material is a button material, the hotpress-stretching can be carried out, for example, with an apparatusshown in FIG. 1.

FIG. 1 shows a partial schematic view of one example of an apparatus forhot press-stretching a button material. First, a plate-shaped jig 2 foradjusting a compression height, which has a space portion having athickness adjusted depending upon a compression ratio, is set on a presslower plate 1, a button material 4 having a predetermined heightdimension is placed in the space portion of the jig 2, then, a pressupper plate is lowered until it reaches a top surface of the buttonmaterial 4, heating is carried out for a predetermined period of time,and further, the button material 4 is press-stretched by applyingpressure, whereby the press upper plate 3 is lowered until it reachesthe top end surface of the plate-shaped jig 2, the material is processedto have a height depending upon a compression ratio, and an intended hotpress-stretched material can be obtained.

When the plate-shaped jig having the space portion has a constantthickness, the height of the button material is adjusted depending upona compression ratio, and similar procedures are carried out, whereby anintended hot press-stretched material is obtained.

When the contact lens material is a rod material, the hotpress-stretching can be carried out, for example, with an apparatusshown in FIG. 2 or 3.

FIG. 2 shows a partial schematic view of one example of an apparatus forhot press-stretching a rod material. First, a cylindrical jig 5 foradjusting a compression height is set on a press lower plate 1, a rodmaterial 6 is placed in the center of the jig 5, then, a press upperplate 3 is lowered until it reaches the top end surface of the rodmaterial 6, heating is carried out for a predetermined period of time,and further, the rod material 6 is press-stretched by applying pressure,whereby the press upper plate 3 is lowered until it reaches the upperend surface of the cylindrical jig 5, and an intended hotpress-stretched material is obtained. In this case, when a cylindricaljig having a constant inner diameter is used, it is required to adjustthe outer diameter and height of the rod material such that the outerdiameter of the hot press-stretched material equals the inner diameterof the cylindrical jig. When a rod material having a fixed outerdiameter and a fixed height is used, it is required to adjust the innerdiameter and height of the cylindrical jig such that the outer diameterof the hot press-stretched material equals the inner diameter of thecylindrical jig.

FIG. 3 shows a partial schematic view of another example of theapparatus for hot press-stretching a rod material. First, a rod material6 is placed in the hollow portion of a cylindrical (hollowcolumn-shaped) jig 7, and then, a press member 8 is lowered until itreaches the upper end surface of the rod material 6 [(a)]. Then, the jig8 is heated to maintain the rod material 6 at a predeterminedtemperature, and the press member 8 is lowered such that the rodmaterial 6 has a predetermined compression ratio [(b)]. Thereafter,while the press-stretching is controlled so as to show a predeterminedpressure, the rod material 6 is maintained at the above temperature fora predetermined period of time, and then the jig 7 is cooled by acooling method such as air-cooling with a fan, to cool the compressedrod material to a temperature near room temperature. Then, thecompressed rod material is taken out of the jig 7, to give an intendedhot press-stretched material. It is required to adjust the innerdiameter of the above jig or the diameter of the rod material such thatthe outer diameter of the hot press-stretched material equals the innerdiameter of the cylindrical jig at a predetermined compression of ratio.

Then, the thus hot press-stretched crosslinked gas-permeable hardcontact lens material is machined, e.g., cut and polished, to give agas-permeable hard contact lens improved in durability strength. Themethod of machining such as cutting and polishing is not speciallylimited, and a method generally practiced in the production of aconventional hard contact lens can be employed.

When the above circular button-shaped or rod shaped contact lensmaterial is hot press-stretched, the hot press-stretched material hasoptical strains in some cases. However, the hot press-stretched materialis cut and polished as described above, whereby there can be obtained agas-permeable hard contact lens which is free of optical strains andexcellent in transparency and of which the internal stress is small andthe durability strength is high.

Further, the present invention provides a gas-permeable hard contactlens obtained by the above production process. Generally, thegas-permeable hard contact lens obtained by the above process has acompression ratio in the range of from 5 to 50% and acompression-flexure fracture strength in the range of from 300 to 1,500g.

The above compression-flexure fracture strength refers to a valuemeasured in the following compression-flexure flexure strength testusing a universal tester model 4310 supplied by Instron Co. A samplecontact lens is fixed to upper and lower anvils with water drops, andwhen the anvil was lowered at a constant rate (200 mm/minute), thecontact lens is measured for a compression-flexure fracture strength(g).

Further, the present invention also provides a gas-permeable hardcontact lens formed of a crosslinked gas-permeable hard contact lensmaterial which is hot press-stretched to have the above properties,i.e., a compression ratio of 5 to 50% and a compression-flexure fracturestrength of 300 to 1,500 g.

The present invention will be explained further in detail with referenceto Examples hereinafter, while in present invention shall not be limitedby these Examples.

EXAMPLE 1

A monomer mixture containing 43 parts by weight of 2,2,2-trifluoroethylmethacrylate, 43 parts by weight oftris(trimethylsiloxane)-γ-methyacryloxypropylsilane, 2 parts by weightof a siloxane oligomer having a polymerizable functional group in amolecular terminal, 8 parts by weight of methacrylic acid, 2 parts byweight of dimethylacrylamide, 2 parts by weight of ethylene glycoldimethacrylate as a crosslinking agent and 0.25 part by weight ofazobisisobutyronitrile as a polymerization initiator was polymerized ina pipe formed of polyethylene having an inner diameter of 15 mm, toprepare a crosslinked gas-permeable contact lens material.

Then, the above contact lens material was centerless ground, andbutton-shaped materials (“button materials” hereinafter) each having aheight adjusted to have a compression ratio of 5, 10, 15, 20 ,25 or 50%were prepared with a hydraulic lathe,

Then, each button material having the above adjusted height was hotpress-stretched with an apparatus shown in FIG. 1 as follows.

First, a button material 4 was placed in a space portion of a 5.7 mmthick plate-shaped jig 2 for adjusting a compression height set on apress lower plate 1, then, a press upper plate 3 was lowered so as tosandwich the button material, the button material was heated to 120° C.and maintained at the same temperature for 30 minutes. Then, the buttonmaterial was compression-stretched with a pressure of 25 kg/cm², andwhile the pressure of 25 kg/cm² was maintained, water was circulated inthe apparatus to cool the compression-stretched material to roomtemperature. Then, the pressure was released, the press upper plate 3was elevated, and the hot stretched material was taken out.

In the above manner, press-stretched crosslinked gas-permeable hardcontact lens materials each having a compression ratio of 5, 10, 15, 30,25 or 50% were obtained.

Table 1 shows relationships between heights and diameters of thesematerials before the press-stretching and those after thepress-stretching at each compression ratio.

TABLE 1 Before press- After press- stretching (mm) stretching (mm)Compression Height of Diameter Height of Diameter ratio of button buttonof button button of button material (%) material material materialmaterial 5 6.00 14.00 5.70 14.4 10 6.33 14.8 15 6.71 15.2 20 7.13 15.725 7.60 16.2 50 11.40 19.8

(Height of plate-shaped jig 5.7 mm)

Then, each of the above materials was cut and polished to a givecrosslinked gas-permeable hard contact lens having a thickness ofapproximately 0.15 mm.

EXAMPLE 2

A monomer mixture having the same composition as that used in Example 1was polymerized in a pipe formed of a polyethylene having an innerdiameter of 14, 15, or 16 mm, to prepare crosslinked gas-permeablecontact lens materials. These contact lens materials were cut to give arod-shaped material {circumflex over (1)} having a diameter and a heightof 14.0 mm and 66.7 mm, a rod-shaped material {circumflex over (2)}having a diameter and a height of 15.0 mm and 72.6 mm or a rod-shapedmaterial {circumflex over (3)} having a diameter and a height of 16.0 mmand 63.8 mm (“rod materials” hereinafter).

Each rod material was hot press-stretched with an apparatus shown inFIG. 2 as follows.

First, the above rod material {circumflex over (1)}, {circumflex over(2)} or {circumflex over (3)} was placed in the center of acompression-height-adjusting cylindrical jig 5 set on a press lowerplate 1, the cylindrical jig 5 having an inner diameter/a height of 17.0mm/45.2 mm, 17.0 mm/56.5 mm or 17.0 mm/56.5 mm. Then, a press upperplate 3 was lowered until it reached the top end surface of the rodmaterial 6 to sandwitch the rod material, and the rod material 6 washeated to 120° C. The rod material was maintained at the abovetemperature for 30 minutes. Then, the rod material wascomppression-stretched with a pressure of 25 kg/cm², and while pressureof 25 kg/cm² was maintained, water was circulated in the apparatus tocool the compression-stretched material to room temperature. Then, thepressure was released, the press upper plate 3 was elevated, and thepress-stretched material was taken out of the cylindrical jig 5.

In the above manner, press-stretched crosslinked gas-permeable hardcontact lens materials each having a compression ratio of 11.4, 22.1 or32.2% were obtained.

Table 2 shows relationships between heights and diameters of thesematerials before the press-stretching and those after thepress-stretching at each compression ratio.

TABLE 2 Before press- After press- stretching (mm) stretching (mm)Compression Height of Diameter Height of Diameter ratio of rod rod ofrod rod of rod material (%) material material material material 11.463.8 16.0 56.5 17.0 22.1 72.6 15.0 56.5 32.2 66.7 14.0 45.2

Then, each of the above materials was cut and polished to givecrosslinked gas-permeable hard contact lenses having a thickness ofapproximately 0.15 mm.

EXAMPLE 3

A monomer mixture having the same composition as that used in Example 1was polymerized in a pipe formed of a polyethylene having an innerdiameter of 13 mm, to prepare a crosslinked gas-permeable contact lensmaterial. The contact lens material was cut to give a rod materialhaving a diameter of 13 mm and a height of 500 mm.

The above rod material was not press-stretched with an apparatus shownin FIG. 3 as follows.

First, the above rod material 6 was placed in a hollow portion of acylindrical jig 7 having an inner diameter of 14.5 mm, and then a pressmember 8 was lowered until it reached the top surface of the rodmaterial 6. Then, the jig 7 was heated to increase the temperature ofthe rod material 6 to 120° C. in 20 minutes, and the rod was maintainedat this temperature for 43 minutes. Then, the press member 8 was loweredat a rate of 100 mm/minute (approximately for 1 minute) until the rodmaterial had a compression ratio of 20%. Thereafter, while the pressingpressure was controlled at 3 kgf/cm², the compressed rod material wasmaintained at 120° C. for approximately 2 minutes, and then air-cooledwith a fan over approximately 5 hours until it had a temperature of 40°C. The above press-stretched material was taken out of the cylinder jig7.

The above procedures gave a crosslinked gas-permeable hard contact lensmaterial which was press-stretched to have a compression ratio of 20%and had an outer diameter of 14.5 mm and a height of 400 mm.

Then, the above material was cut and polished to give a crosslinkedgas-permeable hard contact lens having a thickness of approximately 0.15mm.

Comparative Example 1

A monomer mixture having the same composition as that used in Example 1was polymerized to obtain a contact less material, and the contact lensmaterial was cut and polished to prepare a crosslinked gas-permeablehard contact lens material without hot press-stretching it.

Test Example 1

Compression-flexure Fracture Strength Test and Observation of FractureForm

The press-stretched contact lens material having different compressionratios, obtained in Examples 1 and 2, the press-stretched contact lensmaterial obtained in Example 3 and the press-stretching-free contactlens material obtained in Comparative Example 1 were processed to acontact lens form of BC 7.80, POW-3.00 and DIA 8.8, and the so-preparedcontact lenses were subjected to the following compression-flexurefracture strength test with a universal tester model 4310 supplied byInstron Co.

That is, each contact lens was separately fixed to upper and loweramvils with water drops, and the anvil was lowered at a constant rate(200 mm/minute) to determine a compression-flexure fracture strength(g), a compression-flexure fracture deformation (mm), acompression-flexure fracture deformation ratio (%) and a compressionflexural strength (g) at a 30% deformation. Further, each contact lenswas observed for a fracture form.

Table 3 shows the results.

TABLE 3 Comp.- Comp. Comp.- flexure flexural Comp.- flexure fracturestrength flexure fracture defor- at 30 % Comp. fracture defor- mationdefor- Observation ratio strength mation ratio mation of fracture Sample(%) (g) (mm) (%) (g) form CEx. 1 — 224.1 6.10 69.3 65.6 Divided topieces Ex. 1 25 1,303.0 8.27 93.9 66.0 Divided to 2-3 pieces 50 1,413.08.32 94.5 63.4 Not divided, cracked Ex. 2 11.4 655.7 7.98 90.7 66.3Divided to 2-6 pieces 22.1 1,280.0 8.23 93.6 67.5 Divided to 2-4 pieces32.2 1,403.0 8.20 93.2 69.3 Divided to 2-4 pieces Ex. 3 20 900 7.83 89.071.8 Not divided Comp. = Compression, Ex. = Example, CEx. = ComparativeExample

As shown in Table 3, there are great differences observed in thecompression-flexure fracture strength depending upon whether the hotpress-stretching is carried out of not. With an increase in thecompression ratio, the compression-flexure fracture strength increases.

With an increase in the compression ratio and with an increase in thecompression-flexure fracture strength, the compression-flexure fracturedeformation increases. This means that the distance until the lens isfractured increases when the anvil is lowered at a constant rate, and itis therefore assumed that the lens deflection increases.

It is assumed that the above tendency is produced because the molecularchains of the crosslinked gas-permeable contact lens materials come tohave an oriented molecular structure by the hot press-stretching.

Test Example 2

Oxygen Permeability Measurement

The press-stretch contact lens materials having different compressionratios, obtained in Example 1, and the press-stretching-free contactlens material obtained in Comparative Example 1 were processed into adisk form having a diameter of 14 mm and a thickness of 0.25 mm, and theso-prepared disks were measured for degrees of oxygen permeability witha Seika-ken film oxygen permeation meter K316 supplied by Rika SeikiKogyo, K.K., and measure values are shown in terms of electric currentvalues. A higher current value shows better oxygen permeability. Table 4shows the results.

TABLE 4 Ratio to Compression Thickness Current Comparative Sample ratio(%) (mm) value (μA) Example 1 CEx. 1 — 0.25 43.9 — Ex. 1 5 0.25 44.91.018 10 0.24 44.7 0.973 15 0.25 43.9 1.000 20 0.25 43.9 1.000 25 0.2543.9 1.000 50 0.26 40.1 0.946 Ex. = Example, CEx. = Comparative Example

As shown in Table 4, it is seen that the press-stretched contact lensmaterials having a compression ratio of 5 to 25% have almost the sameoxygen permeability as that of the press-stretching-free contact lensmaterial.

Test Example 3

Observation of Optical Strains

The press-stretched contact lens materials having different compressionratios, obtained in Examples 1 and 2, and lens-shaped articles preparedby mechanically processing these materials were respectively observedfor optical strains through a stereoscopic microscope with a polarizerby a crossed Nicol prism method, and the materials and articles wereevaluated on the basis of the following ratings. Table 5 shows theresults.

TABLE 5 Compression Example 1 Example 2 ratio (%) 5 10 15 20 25 50 11.422.1 32.2 Material ∘ x x x x x x x x Lens ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ No opticalstrain is observed. ∘ A slight optical strain is observed. x An opticalstrain is easily observed.

The contact lens materials showed radial optical strains when they had acompression ratio of 10% or more. However, the lenses obtained bycutting and polishing the press-stretched materials showed no opticalstrains.

Test Example 4

Stability Test

The press-stretched contact lens having a compression ratio of 5, 25, or50%, obtained in Example 1 and, the press-stretching-free contact lensobtained in Comparative Example 1 were respectively inserted in inwet-lens cases filled with a storage solution, and they were stored atroom temperature for 1 month and in a constant-temperatureconstant-humidity chamber (supplied by Kusumoto Kasei K.K.) underconditions of 40° C. of temperature and 70% of humidity for 2 months.Then, the lenses were measured for changes of base curves, diopters andthicknesses with the passage of time, and these were compared with theirinitial values. Further, the above contact lenses were respectivelyobserved for optical strains through a stereoscopic microscope with apolarizer by a crossed Nicol prism method before and after the test.

Tables 6 and 7 show the results.

TABLE 6 Comp. Amount ratio Items measured or Initial After 1 of Sample(%) observed value month change CEx. 1 — Base curve (mm) 7.81 7.81 0Diopter (D) −3.19 −3.23 −0.04 Thickness (mm) 0.161 0.159 −0.002 Opticalstrains No No No Ex. 1 5 Base curve (mm) 7.82 7.81 0.01 Diopter (D)−3.42 −3.47 −0.05 Thickness (mm) 0.148 0.145 −0.003 Optical strains NoNo No 25 Base curve (mm) 7.82 7.83 0.01 Diopter (D) −3.32 −3.30 0.02Thickness (mm) 0.155 0.155 0 Optical strains No No No 50 Base curve (mm)7.81 7.81 0 Diopter (D) −2.67 −2.69 −0.02 Thickness (mm) 0.165 0.165 0Optical strains No No No Comp. ratio = Compression ratio, CEx. =Comparative Example, Ex. = Example, (Stored at room temperature for 1month)

TABLE 7 Items Comp. measured Amount Sam- ratio or Initial After 1 After2 of ple (%) observed value *1 month *2 months *3 change CEx. — Base7.81 7.81 7.80 −0.01 1 curve (mm) Diopter −3.23 −3.20 −3.25 −0.02 (D)Thickness 0.159 0.161 0.158 −0.001 (mm) Optical No No No No strains Ex.1 5 Base 7.81 7.83 7.82 0.01 curve (mm) Diopter −3.47 −3.44 −3.47 0 (D)Thickness 0.145 0.149 0.148 0.003 (mm) Optical No No No No strains 25Base 7.83 7.82 7.81 −0.02 curve (mm) Diopter −3.30 −3.32 −3.31 −0.01 (D)Thickness 0.155 0.156 0.153 −0.002 (mm) Optical No No No No strains 50Base 7.81 7.80 7.79 −0.02 curve (mm) Diopter −2.69 −2.69 −2.70 −0.01 (D)Thickness 0.165 0.164 0.164 −0.001 (mm) Optical No No No No strains *1Results after storage at room temperature for 1 month (results afterstorage for 1 month in Table 6) *2 Results after storage at roomtemperature for 1 month and further in an environment having atemperature of 40° C. and a humidity of 70% for 1 month. *3 Resultsafter storage at room temperature for 1 month and further in anenvironment having a temperature of 40° C. and a humidity of 70% for 2months. Comp. ratio = Compression ratio, CEx. = Comparative Example, Ex.= Example

As shown in Tables 6 and 7, when the lens were stored at roomtemperature for 1 month or stored in an environment having a temperatureof 40° C. and a humidity of 70% for 2 month, almost no changes wereobserved. Further, when the lenses were observed for optical strains, nogeneration of strains were found.

The results of the above stability test show that the gas-permeable hardcontact lens of the present invention is a gas-permeable hard contactlens of which the internal stress is vary small.

EXAMPLES 4-6

A button-shaped contact lens material was prepared in the same manner asin Example 1 except that the amounts of monomers were changed as shownin Table 8, and then, the button material was not press-stretched in thesame manner as in Example 1 to give a crosslinked gas-permeable hardcontact lens material having a compression ratio of 5%, 25% or 50%.

Then, each of the above contact lens materials was processed into a lensform in the same manner as in Test Example 1, and each lens was testedfor compression-flexure fracture strength. Table 9 shows the result.

TABLE 8 Example 4 Example 5 Example 6 Amount FMA 43 47 50 ratio SIMA 4330 25 (part by MA 5 10 10 weight) DAA 3 5 5 EDMA 3 3 5 Siloxane oligomer3 5 5 (Notes) FMA: 2,2,2-trifluoroethyl methacrylate SIMA:tris(trimethylsiloxane)-γ-methacryloxypropylsilane MA: mthacrylic acidDAA: Dimethylacrylamide EDMA: Ethylene glycol dimethacrylate Siloxaneoligomer: siloxane oligomer having a polymerizable functional group in amolecular terminal.

TABLE 9 Compression ratio Compression-flexure (%) fracture strength (g)Example 4 5 558.0 25 1,009.0 50 1,323.7 Example 5 5 340.1 25 1,059.0 501,363.8 Example 6 5 443.6 25 1,015.0 50 1,339.7

Industrial Utility

According to the process of the present invention, there can beefficiently produced a gas-permeable hard contact lens which is free ofoptical strains, excellent in transparency, of which the internal stressis small and further which has no problem on wearing and is improved indurability strength.

What is claimed is:
 1. A process for the production of a gas-permeablehard contact lens obtained by heat polymerization or ultravioletpolymerization, which comprises hot press-stretching a crosslinkedgas-permeable hard contact lens material and then machining thepress-stretched material.
 2. The process of claim 1, wherein thecrosslinked gas-permeable hard contact lens material is a copolymerformed from a fluorine-containing (meth)acrylate and asilicon-containing (meth)acrylate as main components.
 3. The process ofclaim 2, wherein the fluorine-containing (meth)acrylate is fluoroalkylmethacrylate.
 4. The process of claim 2, wherein the silicon-containing(meth)acrylate is siloxanyl methacrylate.
 5. The process of claim 2,wherein the copolymer has a fluorine-containing (meth)acrylateunit:silicon-containing (meth)acrylate unit content ratio of 70:30 to40:60 by weight.
 6. The process of claim 2, wherein the crosslinkedgas-permeable hard contact lens material is a copolymer which furthercontains a crosslinked monomer component.
 7. The process of claim 6,wherein the copolymer has a crosslinking monomer unit content of 0.1 to20% by weight.
 8. The process of claim 2, wherein the crosslinkedgas-permeable had contact lens material is a copolymer which furthercontains a hydrophilic monomer component.
 9. The process of claim 8,wherein the copolymer has a hydrophilic monomer unit content of 5 to 20%by weight.
 10. The process of claim 2, wherein the crosslinkedgas-permeable hard contact lens material is a copolymer which furthercontains a siloxane oligomer component having a polymerizable functionalgroup in a molecular terminal.
 11. The process of claim 10, wherein thesiloxane oligomer having a polymerizable functional group in a molecularterminal is a compound of the general formula (I),

wherein m is an integer of 5 to 200, and each of A and A′ is a group ofthe general formula,

in which R is a hydrogen atom or methyl, and may be the same as theother of different from the other.
 12. The process of claim 10, whereinthe copolymer contains 0.1 to 15% by weight of the siloxane oligomercomponent having a functional group in a molecular terminal.
 13. Theprocess of claim 1, wherein the crosslinked gas permeable hard contactlens material has the form of a circular button or a rod and is notpress-stretched to have a thickness smaller that a thickness which thecrosslinked gas-permeable hard contact lens material has before it ishot press-stretched.
 14. The process of claim 13, wherein the hotpress-stretched is carried out to attain a compression ratio of 5 to50%.
 15. The process of claim 13, wherein the crosslinked gas-permeablehard contact lens material having the form of a circular button or a rodhas a temperature of 100 to 140° C. during the hot press-stretching. 16.A gas-permeable hard contact lens obtained by the process recited inclaim
 1. 17. The gas-permeable hard contact lens of claim 16, which hasa compression ratio of 5 to 50% and a compression-flexure fracturestrength of 300 to 1,500 g.
 18. A gas-permeable hard contact lens formedof a crosslinked gas-permeable hard contact lens material which is hotpress-stretched to have a compression ratio of 5 to 50% and acompression-flexure fracture strength of 300 to 1,500 g.
 19. Thegas-permeable hard contact lens of claim 18, wherein the crosslinkedgas-permeable hard contact lens material is a copolymer formed from afluorine-containing (meth)acrylate and a silicon-containing(meth)acrylate as main components.
 20. A process for the production of agas-permeable hard contact lens, which comprises hot press-stretching acrosslinked gas-permeable hard contact lens material, wherein thecrosslinked gas-permeable hard contact lens material has the form of acircular button or a rod and is hot press-stretched to attain acompression ratio of 5 to 50% to have a thickness smaller than athickness which the crosslinked gas-permeable hard contact lens materialhas before being hot press-stretched, and thereafter machining thepress-stretched material.